Dye-sensitized solar cells are being researched and developed actively as next generation solar cells that can substitute for silicon-based solar cells, because dye-sensitized solar cells have excellent advantages. Such advantages are as follows: (1) they are expected to need production costs as low as ⅕ to 1/10 of those of silicon-based solar cells and therefore they are inexpensive, (2) the amount of CO2 emitted during their production is as small as 1/10 or less of that with single crystal silicon solar cells, (3) the energy payback time or the CO2 payback time thereof is as short as half or less of those of polycrystalline silicon solar cells, (4) there are fewer constraints with respect to resources of their raw materials, (5) they excel in aesthetic properties and processability and therefore larger-sized ones can be manufactured easily, and (6) they have relatively high photoelectric conversion efficiencies as high as 10% or more, which are comparable to those of amorphous silicon solar cells.
FIG. 1, FIG. 3, and FIG. 4 each shows schematically the sectional configuration of an example of the conventional dye-sensitized solar cells. FIG. 2 extracts and enlarges a principal part of the dye-sensitized solar cell represented in FIG. 1. The dye-sensitized solar cell represented in FIG. 1 is a product in which a working electrode 10 and a counter electrode 20 are opposed to each other via an electrolyte-containing layer 30. At least one of the working electrode 10 and the counter electrodes 20 is a light-transmissive electrode. The working electrode 10 has a substrate 11A on which a conductive layer 11B and a metal oxide semiconductor layer 12 are laminated together. The dye-sensitized solar cells depicted in FIG. 3 and FIG. 4 are of interest because they can be produced at low cost since the conductive layer 11B needs not be transparent if the substrate 11A is transparent. When the configuration of FIG. 3 or FIG. 4 is adopted, the conductive layer 11B to be used need to be porous or lattice-like one through which electrolyte components can pass (Non-Patent Literatures 1 and 2).
As shown in FIG. 1 and FIG. 2, a dye-sensitized solar cell is composed of a working electrode (photoelectric conversion device) 10, a counter electrode 20, and an electrolyte-containing layer (electrolytic solution) 30 sandwiched between the two electrodes. The working electrode 10 is prepared by following procedure. First, nanosized metal oxide semiconductor particles 12B are applied to a surface of a substrate 11A such as glass on the conductive layer side on which a conductive layer 11B is formed. Next, the metal oxide semiconductor particles 12B are baked to form a metal oxide semiconductor layer 12. Then, a dye 13 is fixed to the metal oxide semiconductor particles 12B by chemical/physical adsorption. The counter electrode 20 is a product in which a conductive layer 22 is formed on a surface of a substrate 21 such as glass. The counter electrode 20 is prepared by applying platinum treatment or conductive carbon treatment in a catalytic amount to the conductive layer side of the substrate 21. The solar cell is prepared by superposing the working electrode 10 and the counter electrode 20 and then injecting the electrolyte composition containing an iodine compound (electrolyte-containing layer 30) to between the electrodes 10 and 20.
In the power generation mechanism of a dye-sensitized solar cell, electrons are injected from a sensitizing dye excited by solar light (visible light) irradiation into a conduction band of a metal oxide semiconductor. The injected electrons are introduced into an external circuit through a photoelectrode and then move to a counter electrode and then, via a redox reaction of an electrolyte composition, the sensitizing dye (dye cation) in an oxidized state receives electrons to regenerate. Photoelectric conversion is attained by this cycle.
Because of a lower photoelectric conversion efficiency as compared with the commercially available silicon-based solar cells, dye-sensitized solar cells are not in industrial use. The main factor of the drop of photoelectric conversion efficiency of a dye-sensitized solar cell lies in the drop of voltage caused by reverse electron transfer from an oxide semiconductor layer to an electrolyte composition and a dye cation and therefore addition of a basic compound to an electrolyte composition has been investigated in order to inhibit reverse electron transfer to prevent voltage drop.
The addition of a basic compound inhibits reverse electron transfer, but it has a problem that the sensitizing dye adsorbed on the metal oxide semiconductor is desorbed easily. Thus, there have been attempts at inhibiting the desorption from a metal oxide by making a sensitizing dye have an anchor group such as a carboxyl group and a silanol group (Patent Literature 1 to 3).